Up-regulation of Amazon forest photosynthesis precedes elevated mortality under drought

Abstract:

Coupled climate-carbon cycle models indicate that Amazon forests may be vulnerable to drought, with some predicting drought-induced collapse of the Amazon forest and conversion to savanna, under future climate change. While much progress has been made in understanding tropical forest drought response, a holistic picture encompassing both short-term physiological (e.g., photosynthesis) and longer term demographic responses (e.g., mortality) remains elusive, mainly due to the rarity of coinciding relevant measurements and drought events. Here we address this knowledge gap by analyzing the response of an eastern Amazonian forest at both timescales to the El Nino-induced drought of late 2009 / early 2010 (distinct from the Atlantic SST-induced drought to hit Western Amazonia in mid/late 2010) using eddy flux measurements of carbon exchange and periodic tree demographic surveys. We partitioned the drought response of GPP into environmental (light, vapor pressure deficit (VPD), diffuse light) and biological responses. Based on environmental conditions alone (high VPD), we expected GPP to be 1-2 umol CO₂ m^-2 s^-1 less than average during drought. In contrast, GPP was elevated by 2-4 umol CO₂ m^-2 s^-1 above this expected negative response over a period of ~45 days, consistent with previously observed green-up from satelliltes during the 2005 Amazon drought. At the same time, drought significantly elevated 2009-2011 tree mortality, by ~50% above that during non-drought periods, consistent with a quantitative drought-mortality relationship reported for other Amazon forests. This work suggests that observations of “green-up” of forest canopies during drought may be consistent with subsequent drought-induced tree mortality, in contrast to expectation. More importantly, it highlights endogenous biological regulation of photosynthesis as an important mechanism, neglected by models, in mediating drought responses in tropical forests.